On average, the amount of parking provided at mixed-use centers is 65 percent higher than necessary, according to a study just published in the Transportation Research Record. That finding challenges the perceived need for additional parking in many of those places. Meanwhile, the unused parking spaces take up valuable space, add to construction and maintenance costs, and undermine efforts to manage travel demand.

The study looked at 27 districts, mostly in California and Massachusetts, where transportation consultants from Nelson/Nygaard had conducted past parking studies. Assuming that an ideal lot should be 85% full at peak, the researchers tallied up extra stalls at each study location.

Overall, parking was oversupplied by between 6 and 253 percent. Nine of the parking studies were initiated because of perceived parking scarcities. Even in those places, oversupplies ranged from 6 to 82 percent, with an average of 45 percent. The authors didn’t find any statistically significant explanatory variables—due partly to the small number of districts in the study—but oversupplies were generally higher in cities versus suburbs, in the western states, and where driving alone was more common.

According to the authors, this finding challenges the principles that many policymakers adhere to in setting parking policies: “[T]he evidence here suggests that levels of parking provision are unmoored from demand, travel behavior, pricing, or other dimensions where theory suggests that there would be a relationship.” By allowing so much parking, they explain, policymakers lose much of the leverage they might have to manage travel demand, chiefly through pricing.

But mixed-use centers are not the only known cases of parking oversupply. A study of residential parking in Washington, DC, found that only 60 percent of stalls are used on average. That number is approximately the same for a study currently underway by SSTI in Madison, Wisconsin. In Chicago and King County, Washington, that number is closer to 70 percent. Using the same definition as the Nelson\Nygaard study, this means that parking is oversupplied by around 20 to 42 percent in those places.